The invention lies in the field of processes for the recovery of aluminum from its ores and/or the manufacture of monobasic potassium phosphate from apatite ore.
The present invention is a combination of the process for producing phosphoric acid from apatite rock by the wet process and the production of aluminum from alunite in accordance with the basic process, the present process resulting in the continuous production of aluminum and monobasic potassium phosphate as the products. In the operation of the process, phosphoric acid produced by the treatment of apatite rock with sulfuric acid is cycled directly to the basic process where it is subjected to electrodialysis with by-product potassium sulfate from the basic process to produce monobasic potassium phosphate.
The basic process is defined by the following process steps, some of which are optional, as illustrated in the flow diagram of the referenced patents:
(1) roasting particulate alunite concentrate to remove water of hydration; PA1 (2) removing sulphur compounds and alkali metal compounds, including potassium sulfate, from the roasted ore either by the procedure of U.S. Pat. No. 3,890,425 in which the roasted ore is subjected to a reducing roast and then to an oxidizing roast followed by leaching with solvent, such as water or an alkali metal hydroxide, or by the process of U.S. Pat. No. 3,890,426 in which the roasted ore is leached in ammonium or alkali metal hydroxide, each alternate procedure terminating in a leaching step and leaving a residue containing aluminum values; PA1 (3) digesting the residue from the leaching step with alkali metal hydroxide solution to convert aluminum values in the residue to soluble aluminate; PA1 (4) precipitating silica from the resulting solution of step (3) to leave a solution of aluminate and a residue known as a desilication product consisting essentially of sodium aluminum silicate along with other impurities; PA1 (5) precipitating aluminum values from the solution of step (4); and PA1 (6) recovering product aluminum from the aluminum values of step (5).
The expression "roasting and leaching to remove water and compounds of sulfur and alkali metals" as used herein includes the two procedures of step (2) above.
The recited basic process includes the alternative of recovering by-product potassium sulfate by crystallization as disclosed in U.S. Pat. No. 3,890,426 in which some potassium sulfate is bled off from the crystallization step. This potassium sulfate bleed stream is included in the term "by-product potassium sulfate" from the basic process referred to herein.
Processes for the production of aluminum and monobasic potassium phosphate must be as economical as possible in order to produce competitive commercial grade products. This necessarily means conducting the production processes with a minimum of process steps and the recovery and utilization of as many by-products as possible. The combination of processes for the production of two or more products with the elimination of processing steps and the recovery and reuse in the combined process of materials which are normally by-products in the single processes is a distinct advantage. This is particularly true in this instance in which the organization producing the products by different processes is the owner of ore bodies containing ores from which the two products are produced by disconnected processes.
In the production of phosphoric acid from apatite rock by the wet process for sale as such or for use in the manufacture of fertilizer, a high purity product is required to make it commercial grade. Concentration and other purification steps are necessary as it cannot be marketed or used in the impure form in which it is produced. The purification procedure is a substantial item of expense in the production of a saleable product. A further problem in the use of the wet process phosphoric acid to manufacture monobasic potassium phosphate is that sulfate contamination of the product is a problem. If the raw phosphoric acid could be used without purification to make a commercial grade product, such as fertilizer which is saleable with a minimum of purification, it would result in a realization of a higher profit for the phosphoric acid itself due to the elimination of the purification steps. This is especially true when the product is monobasic potassium phosphate because its value and marketability is much greater than that of phosphoric acid and potassium sulfate combined.
Likewise, the recovery of potassium sulfate from potassium sulfate solution resulting from the first leaching step of the basic process normally requires evaporation, crystallization, and other procedures to produce a commercial grade product, all of which add to the cost of producing the potassium sulfate for sale. Accordingly, as is true with raw phosphoric acid produced by the wet process, if the impure potassium sulfate could be used to produce a commercial grade product, such as, monobasic potassium phosphate, the profit realized for the by-product potassium sulfate from the basic process would in the end be increased as the expensive purification procedures would be eliminated.
Accordingly, it is an object of this invention to provide a combined process for producing phosphoric acid from apatite rock and aluminum from alunite ore in which process monobasic potassium phosphate is produced from the raw phosphoric acid and raw by-product potassium sulfate from the aluminum production process, with the result that processing steps are eliminated and by-products and normally discarded materials are recovered and re-used in the combined process.